Larry Tankersley Extension Associate Forestry, Wildlife & Fisheries University of Tennessee
Trees benefit us in many ways. They can also cause major damage when limbs or whole trees fall on power lines, cars, houses or people. Usually, weakened trees give some warning signs of danger. By learning to recognize the signs and to follow-up with prompt, proper action, you can often manage this risk, saving yourself grief as well as money.
What is a Hazard Tree?
A tree failure occurs when a tree or large part of a tree breaks and falls. Trees become a potential hazard when there is a target. A target is a structure, vehicle or a person that would be struck by a falling tree or its parts. The target directly influences the degree of hazard. Consider the differences among a tree falling on a fence, a house or a person. A tree leaning over the bedroom is most hazardous. Trees near high-use areas are more of a risk than those near infrequently visited areas, as the probability of a person being hit is greater. Priorities for removal or corrective treatments depend on the hazard rating of the tree.
Tree age is important in hazard tree management. Every tree species has an inherent life span. Risk of failure increases with age. Longevity should be considered when evaluating existing tree hazards or selecting species to plant. Generally, longer-lived species are preferred, unless plans are made to maintain or periodically replace less persistent species. The environment in which a tree lives will also determine its hazard potential.
Observe the Trunk
Decay, a major cause of tree failure, is caused by fungi that weaken wood as they grow and reproduce. As healthy trees bend and sway, wood fibers slide past each other. Decaying tissues, however, are not flexible and often break. The presence of many reproductive structures often indicates advanced stages of decay. Decay is often present without obvious signs. Cracks, seams, butt swell, dead branch stubs and large, older wounds suggest internal decay.
Wounds and cankers are two types of tree defects associated with hazards. Cankers are usually tree diseases that are perennial and aggressive. These defects enlarge with time and increase the likelihood of tree failure. Wounds and cankers can be weak points on a trunk and their position relative to the prevailing winds influences the risk they represent. A tree is more likely to break at a wound or canker if it is facing or opposite to the direction of the prevailing wind. Vertical cracks or seams along the trunk suggest internal defects.
A hollow tree is not necessarily a hazard tree. Cavities develop from bark wounds. Many old trees have large conspicuous cavities. Vigorous trees have been observed to grow more sound wood around the hollow, compensating for that lost to decay. Compartmentalization of the decay also prevents the size of the rotten compartment from expanding.
Inspect the Crown
Crown vigor and form are two indicators of the general health of trees. Crown characteristics of a potential hazard tree include dieback, V-shaped forks and lopsidedness. V-shaped forks are weak compared to broader angled forks and branches.
Branches in the upper crown often die from the top down in response to stress. Repeated insect defoliation, extended periods of drought, soil compaction and/or root disease can cause stress. Opportunistic pests, such as insects and fungi, often invade and further stress the tree. Trees can recover from dieback, if the source of the stress is eliminated in time. However, trees with advanced crown decline will die and should be removed.
Leaning, lopsided trees may represent a hazard if they are above a target. Generally, trees that lean more than 15 degrees from vertical should be removed. Trees that have grown leaning are not as hazardous as trees that were originally straight, but subsequently developed a lean due to wind or root damage. The general growth form of the tree and any uplifted soil on the side of the tree opposite the lean provide clues to when the lean developed.
Root integrity and health cannot be over emphasized. If the roots are damaged in any way, tree vitality and health are affected and the likelihood of failure increases. The probability of failure increases as the amount of the root damage increases. Above-ground clues to poor root conditions include thin crowns, dwarfed, off-color leaves, stunted growth, discolored or resin soaked wood at the root collar and fruiting bodies of root- rot fungi growing at or near the base of the trees.
What Can You Do?
Check your trees, especially large, old ones. Periodic, thorough inspections are essential to prevent accidents. Every tree likely to have a problem should be inspected from bottom to top, looking for signs of root or butt rot and continuing up the trunk toward the crown, noting anything that might indicate a potential hazard. At least one inspection per year should be made, but two per year are recommended, one in the summer while the leaves are on the tree and one in the winter.
Since all trees are potential hazards, the only way to completely eliminate a tree hazard is to remove the tree. Where this is not acceptable, regular inspection and appropriate action is the best way to reduce your risks. Dead trees within the range of a target should be removed. When removing a hazard tree, prevent creating another hazard tree by limiting damage to the site and residual trees.
Prevention is the best action. Start a tree health program as soon as possible. Proper selection and placement of trees prevents many hazard problems.
Hazard trees are unpredictable. They do not always fall when and where you think. Don’t solve one problem just to create another. When in doubt contact your county AgriLife Extension agent or certified tree professional.
Harris, Richard W. 1992. Arboriculture: integrated management of landscape, trees, shrubs, and vines. 2nd ed., Prentice Hall, Inc. 674 p.
Johnson, David. 1981. Tree hazards: recognition and reduction in recreation sites. Technical Report R2-1. Lakewood, CO: USDA Forest Service, Forest Pest Management. 17 p.
Minnesota Dept.of Natural Resources and USDA Forest Service. 1996. How to recognize hazardous defects in trees. NA-FR-01-96. Radnor, PA: USDA Forest Service, Northeastern Area. 20 p.
Robbins, Kathryn. 1986. How to recognize and reduce tree hazards in recreation sites. USDA Forest Service NA-FR-31.
Sharon, E. Michael. 1987. Tree health management: evaluating trees for hazard. Journal of Arboriculture 13(12):285-293.
Tattar, Terry A. 1982. Living hazard-trees. Publication L-264, Cooperative Extension Service, Univ. of Massachusetts, Amherst.